1. Field of the Invention
The present invention relates to a process for preparing a 2,3-di-substituted cyclopentanone mixture and a fragrant composition comprising the mixture which has excellent jasmine flower-like fragrance as well as a diluted fragrant composition.
2. Related Art Statement
2,3-Di-substituted cyclopentanones represented by methyl jasmonate and methyl dihydrojasmonate are chemical substances useful in the fields of fragrance, drugs, chemicals and the like.
Such 2,3-di-substituted cyolopentanonee involve steric isomers of cis-isomer and trans-isomer, as shown by general formulae (I) through (IV) described below. Recent studies reveal that the ois-isomer is excellent in organoleptic evaluation and physiological activity, as compared to the trans-isomer. ##STR1## (wherein R.sub.1 represents a hydrocarbon residue and R.sub.2 represents a hydrocarbon residue which may optionally have a substituent).
In the case of, for example, methyl jasmonate, it is reported that the essence of its fragrance is present in the cis-isomer (Agricultural and Biological Chemistry, 49, 769, 1985) and that the cis-isomer is more active in the physiological activity on plants than the trans-isomer or a mixture of the cis-isomer and the trans-isomer (Preprints of the Plant Physiological Association, page 347, 1989; Plant Physiology, 66, 246, 1980, etc.).
Based on these findings, synthesis of and research on cis-2,3-di-substituted cyclopentanones have been extensively made. The syntheses of, for example, methyl jasmonate or methyl dihydrojasmonate are reported in Preprints of the 29th Discussions on Fragrance, Terpene and Essential oil Chemistry, page 222, 1985; Preprints of the 30th Discussions, page 101, 1986; Preprints of the 32nd Discussions, page 132, 1988; etc. However, these processes fail to efficiently produce the cis-isomer. In addition, the processes encounter practical problems since many reaction steps are involved and expensive reactants are used.
On the other hand, it is known that when the cis-isomer is compared to the trans-isomer, the trans-isomer is generally advantageous in view of equilibrium. It was thus studied to obtain the cis-isomer efficiently by converting the trans-isomer into the cis-isomer and recently, a process for treating methyl jasmonate with p-toluenesulfonic acid in the presence of a solvent was reported (Fragrance Industry News, No. 247, Apr. 15, 1988, page 6). According to the process, however, the yield of the cis-isomer is as low as about 5%. In addition, there is a fear that side reactions tend to occur.
As described above, methyl jasmonate [formula (I-a) below] and methyl dihydrojasmonate [formula (II-a) below] have jasmine flower-like fragrance and are useful compounds as perfume or cosmetics and as fragrance formulated in food. ##STR2## These compounds have two chiral centers (two asymmetric carbons in the 5-membered ring) so that 4 optical isomers are present, respectively.
For example, methyl jasmonate has 4 optical isomers as shown by formulae (III-a) through (VI-a) described below. As stated above, it is revealed that the essence of fragrance is present in the (+) cis-isomer shown by formula (III-a) below, among these isomers. ##STR3## Therefore, synthesis of and investigations on the (+) cis-isomer have been extensively made as described above, which are reported in, for example, Agricultural Biological Chemistry, 5, 1129, 1987. However, these processes involve many reaction steps and use of expensive reactants so that production costs are high and the processes are not economical.
An economical process has been developed but the reaction product is a mixture of the cis-isomer and the trans-isomer shown by the above formulae and also takes a racemic form. In addition, the trans-isomer is produced in an amount larger than the cis-isomer because of its equilibrium advantageously shifted toward the trans-isomer so that the ratio of the cis-isomer to the trans-isomer contained in the reaction product is approximately 5:95 (weight basis), indicating that the trans-isomer is greatly predominant. Thus, the process involves a defect that the intensity of fragrance is not very strong.